1. Field of the Invention
The present invention relates to a fuel injection control system and method for an internal combustion engine.
2. Description of Related Art
There is disclosed in Japanese Patent Application Laid-Open No. 4-303141 a control system for an internal combustion engine. In this system fuel injection end timing is determined before starting an intake stroke or during the intake stroke in response to timing for completely closing an intake valve. Specifically, injection is ended before the start of the intake stroke when the timing for completely closing the intake valve is earlier than a predetermined discriminating timing and is ended during the intake stroke when the valve closing timing is later than the predetermined discriminating timing. Such system allows fuel injection timing to be adequately controlled in response to changes in operating states of the intake valve which occur due to changes in engine load, thus stabilizing the combustion state and improving charging efficiency.
However, because the injected fuel continuously flows into a cylinder until the later period of the intake stroke (until the intake valve closes) in such system, there remains non-vaporized fuel even when the intake valve is closed. Due to that, there is a possibility that the combustion state within the cylinder is worsened and the non-vaporized fuel is exhausted as unburnt HC also increasing wetness of the fuel. Further, because the fuel is not fully vaporized, the heat of vaporization has not been used effectively and efficiency for charging suctioned air or air-fuel ratio mixture is low.
Further, in another system disclosed in Japanese Patent Application Laid-Open No. 3-950, fuel injection amount is increased/decreased in proportion to an increase/decrease of an air amount suctioned to the cylinder so that the air-fuel mixing ratio (air-fuel ratio) of the mixture suctioned into the cylinder is always fixed with respect to crank angle within the intake stroke. This arrangement allows the distribution of the mixture within the cylinder to be homogenized, thus stabilizing combustion.
However, because the mixture is suctioned into the cylinder during the whole period of the intake stroke during which the intake valve is opened in this system, the mixture is suctioned into the cylinder even during a period in which intake flow speed is very low. In such a case, the mixture is not fully homogenized and favorable combustion cannot be realized. As a result, the control of the fuel injection in a lean air-fuel ratio range aiming at the reduction of fuel consumption and of NOx, i.e., the lean-burn control, has not been able to be realized effectively.
Still more, in another system disclosed in Japanese Patent Application Laid-Open NOS. 60-11652 and 60-122239, stratified fuel combustion is carried out by putting timing for injecting fuel by the injector (fuel injection valve) almost at the later half of an intake stroke. Further, in order to suitably realize stratified fuel combustion, the more air amount suctioned to the engine, the higher the pressure of the fuel (fuel pressure) fed to the injector is increased and such that the crankshaft angular position for ending injection is changed corresponding to engine operating states.
However, although this system has allowed stratified fuel combustion to be realized by injecting fuel almost in the later half of the intake stroke, actually the wetness of the port has increased due to the particle size of the fuel injected and supplied from the injector. That is, combustion is incomplete and no adequate stratified mixture is realized because the average fuel particle size is about 150 to 200 .mu.m in general, causing the port to be wet and cylinder to be wet and hardly allowing homogeneous mixture to be provided.